The present invention relates to a process for the manufacture of double-sided and multi-layer printed circuit boards. The method proposed contemplates a specific manufacturing sequence and the use of electroless nickel for providing the necessary interconnections for building the circuitry to the desired thickness and as an etch resist. The method is particularly versatile in reducing the number of steps and variety of chemicals currently necessary to produce these circuit boards.
In the manufacture of printed circuit boards, it is now commonplace to produce printed circuitry on both sides of a planar rigid or flexible insulating substrate. Of increased importance is the manufacture of multi-layer printed circuits which consist of parallel, planar, alternating inner layers of insulating substrate material and conductive metal. The exposed outer sides of the laminated structure are provided with circuit patterns, as with double-sided boards, and the inner layers themselves may contain circuit patterns.
In double-sided and multi-layer printed circuit boards, it is necessary to provide conductive interconnection between and among the various layers and/or sides. This is commonly achieved by providing copper plated through-holes. Copper is provided in various ways such as by electroless or electrolytic deposition or combinations thereof.
In terms of providing the desired circuit pattern on the board, the art has developed a variety of manufacturing sequences, many of which fall into the broad categories of subtractive or additive techniques. Common to the subtractive processes is the need to etch away (or subtract) metal to expose the desired circuit patterns. Additive processes, on the other hand, begin with clean dielectric substrate surfaces and build up thereon metallization in desired areas only, the desired areas being those not masked by a previously applied pattern of plating resist material. While avoiding the problems associated with the etching required in subtractive processes, additive processes have their own inherent difficulties in terms of the choice of resist materials, the ability to build up the full metallization thickness desired by electroless methods, the relatively long time periods required to electrolessly build the desired thickness""and weaknesses in the physical properties of most electroless copper, deposits.
U.S. Pat. No. 4,897,118 (Ferrier et. al), whose teachings are incorporated herein by reference, reveals a process for selective metallization a substrate in a predetermined desired pattern (i.e. additive technology). Ferrier et. al. discussed additive technology, proposed certain improvements thereto, and give a fair picture of the current state-of-the-art in this area. The current invention proposes improvements thereto which provide significant advantages in reducing the number of steps and chemicals involved in the fabrication thereby making the fabrication process more economical and feasible.
The prior art additive processes suffered from a variety of problems. Firstly, most plating masks currently used in the industry are strippable in alkaline solutions. Electroless copper baths are invariably alkaline, usually very alkaline, with pH""s in excess of 12. Therefore, known plating resists have great difficulty in maintaining their integrity and adhesion to the board surface when subjecting to plating in electroless copper baths, particularly when the long plating periods required by these techniques (8 to 24) hours are taken into consideration. When the plating mask loses its integrity and/or adhesion to the surface, circuit definition fails. Thus the current invention""s proposal of the use of a permanent solder mask as both a solder mask and a plating resist overcomes these problems. For a discussion of soldermasks, their composition and uses, see U.S. Pat. No. 5,296,334, the teachings of which are incorporated herein by reference in their entirety. The permanent nature of the solder mask makes it much more resistant to subsequent processing solutions. The application of solder mask prior to the formation of holes in the printed circuit board has not, heretofore, been attempted. As can be seen from this invention disclosure, several benefits flow from this early application of solder mask. Secondly, the plating rates of electroless copper baths are relatively slow, usually averaging about 60 to 80 microinches per hour. In comparison, electroless nickel plating rates are about 5 times faster, averaging about 350 microinches per hour. Thus, the production rate utilizing electroless nickel can be approximately 5 times that of electroless copper.
The present invention proposes an improved process for the manufacture of printed circuit boards. The method provides various advantages over the prior art, including reduced number of cycle steps, reduced number and types of necessary chemical treatments and increased manufacturing efficiency. This method thus overcomes many difficulties experienced with prior methods.
The method currently proposed contemplates a specific manufacturing sequence for the production of printed circuit boards in combination with electroless plating for building circuitry to thickness. The most preferred form of electroless plating in this application is electroless nickel. The following basic production cycle is proposed for implementation of this invention:
1. Form circuitry (either double sided or multi layer package)
2. Apply a solder mask
3. Apply a de-sensitizing mask
4. Drill or punch the desired holes
5. Activate holes
6. Strip away the desensitizing mask
7. Initiate plating (electroless copper or electroless nickel-boron)
8. Electroless Plate to desired thickness (additive electroless copper or electroless nickel-phosphorous)
9. Final finish
Various optional steps may be added to this basic cycle to suit the particular needs of the fabricator. As used herein, and in the claims, copper clad laminate shall include multilayer circuitry packages as well as double sided circuitry packages.
The present method is an improvement upon the additive techniques for the production of printed circuits. The present invention proposes a type of semi-additive technique. The invention addresses many, if not all, of the concerns and problems experienced by past techniques through the use of a specific processing sequence. The processing sequence allows the application of a permanent coating, the solder mask, very early in the stages of printed circuit production as opposed to at the end of the processing. This early application of the solder mask provides several advantages. Firstly, the solder mask acts as a resist in the electroless plating. Because the solder mask is a permanent casting, it easily maintains adhesion and resists the various processing solutions, whereas the prior art temporary resists frequently lost adhesion in these types of processing sequences. Secondly, the solder mask acts both as a solder mask and a plating resist, thus providing efficiency.
The present invention proposes the following basic cycle for the production of double-sided and multilayer printed circuit boards:
1. Form circuitry (either double sided or multilayer package)
2. Apply a solder mask
3. Apply a desensitizing mask
4. Drill or punch the desired holes
5. Activate holes
6. Strip away the desensitizing mask
7. Initiate plating
8. Plate to thickness
9. Optionally, final finish
The first step calls for the formation of circuitry. In the case of a double sided board, this step would begin with copper clad laminate material followed by the following print and etch sequence. The copper clad laminate material is then imaged on both sides with an etch resist such that the desired circuitry is covered by the etch resist and the remainder of the copper is exposed. The material is then subjected to an etchant such that the exposed copper is etched away allowing the resist covered copper circuitry to stand out in vertical relief. The etch resist is then stripped away revealing the defined copper circuitry on the epoxy-glass laminate. In multilayer applications several of these circuit patterns are laminated together to yield a multilayer package of circuitry with inner layers and outlayers.
In the second step the double-sided or multilayer circuitry from the first step is then coated on its outer surfaces with a permanent solder mask coating. The solder mask can be applied in several ways including dry film, roller coating, curtin coating, screening, or various similar techniques. Generally the solder mask is imaged so that various areas of connection including holes, pads, lands, tabs and similar features (collectively xe2x80x9careas of connectionxe2x80x9d) are exposed. These areas of connection are copper areas where components are later connected to the board or other connections are made to the board. The solder mask can be imaged in various ways including screening, photoimaging followed by development, or similar techniques.
Next, a desensitizing mask is coated on top of the solder mask. The desensitizing mask is imaged such that all of the solder mask is covered, but such that the areas of connection and holes are left exposed. The desensitizing mask can be any non-permanent resist which can be appropriately applied, leaving the areas of connection exposed, and which is resistant to the subsequent processing steps.
The boards are then drilled or punched with holes or vias. Thus, holes will penetrate through the entire board including the solder mask and desensitizing mask on both exterior surfaces. Vias may penetrate through the desensitizing mask and solder mask on one side of the board into the interior of the board but not through to the other side.
Next, the holes are activated to accept plating. Activation of the holes can range in complexity from a single dip in a precious metal activator (or other non-precious metal activates known in the art) to a full DESMEAR (or etch bath), plated though-hole cycle involving numerous steps. The most complex hole activation cycle might consist of hold condition (m-Pyrol), potassium permanganate desmear, neutralization (acid/reducer), glass etch (Ammonium Bifluoride)), conditioner (surfactant or other type), microetch, activator (PdCl2/SnCl2 Colloid) and accelerator. Clean water rinses are interposed between each chemical treatment. Various combinations will be apparent to those skilled in the art. Regardless of which hole activation cycle is chosen, its primary purpose is to treat the holes so that the hole surfaces will initiate electroless plating. A wide variety of methods for achieving this are known in the art, any of which may be advantageously be utilized here. Please refer to U.S. Pat. No. 5,032,427 (Kukanskis et al.), U.S. Pat. No. 4,976,990 (Bach et al), U.S. Pat. No. 4,608,275 (Kukanskis et al.) and U.S. Pat. No. 4,863,758 (Rhodenizer), the teachings all of which are incorporated herein by reference in their entirety.
Once activation of the holes is complete, the desensitizing mask is stripped from the surface of the boards. The particular stripping solution for the desensitizing mask will depend upon the specific mask used. Many non-permanent masks are easily stripped in alkaline solution. Non-permanent masks or resists which can be used as the desensitizing masks in this invention are widely known and utilized in the industry for a wide range of other purposes. Typical desensitizing masks might include dry film masks (resists) available from Dupont Company, Hercules Company or Dynachem, inc. In addition screen printable or liquid photoimageable non-permanent masks are also available from such companies as MacDermid, Incorporated or Ciba-Geigy.
The next step is to initiate plating in the holes and possibly upon the areas of connection. This initiation can occur in several ways. One preferred example is through the application of electroless copper. Another is through the application of electroless nickel-boron. Either of these solutions will initiate plating upon the activated areas only. Thus, the holes and other areas of connection will be plated but the remainder of the unactivated (desensitized) solder mask surface will not.
The next step could be a continuation of the previous step, or, it could be a separate step as indicated on the former sequence. The object of this step is to plate the holes and other areas of connection to the appropriate metal thickness. Thus, if an appropriate electroless copper is used in the previous step, the board could be left in the electroless copper for an extended time to build the appropriate thickness of copper. One preferred method, however is to initiate plating in the previous step with electroless copper (10 to 150 microinches of copper) and then to follow that initiation with either electroless nickel phosphorous or a strike of electroless nickel-boron followed by electroless nickel-phosphorous.
The final step is optional, but recommended. This step consists of applying some form of final finish to the holes and other areas of connection. These final finishes have as their objective, the protection and/or enhancement of the solderability of these surfaces. A final finish may take one of many forms. It may consist of an organic treatment which preserves and enhances the solderability of these surfaces, such as is described in U.S. Pat. No. 5,362,334 (Adams et al.), the teachings of which are incorporated herein by reference in their entirety. Alternatively it may consist of a series of metallic treatments, possibly culminating in a precious metal coating as described in U.S. Pat. Nos. 5,235,139, the teachings of which are both incorporated herein by reference in their entirety.
Various additional steps may be inserted between the steps of the proposed process sequence. In addition, substitutions may also be made. These insertions or substitutions may be such as would be obvious to one skilled in the art.